This invention relates to a semiconductor device and a manufacturing method thereof; and more particularly to the formation of an isolation structure which is employed for isolating active devices from each other in a MOS type VLSI circuit on a Si substrate, and gives a suitable method for manufacturing a fine pattern and large scale integrated device.
In the process of fabricating an LOCOS film (region) in a MOS type VLSI circuit, a double layer consisting of a SiO.sub.2 film and a silicon nitride film which are formed on a Si substrate is used. And the silicon nitride film on an area to be oxidized for isolation is etched away. The exposed area is subjected to heat treatments in an oxidation atmosphere to form an LOCOS film. Thereafter, such bird's beak of LOCOS film as to intrude beneath the silicon nitride film are formed at the edge of the SiO.sub.2 film. The width of bird's beak region amounts to an under value of 0.9-1.0 .mu.m, which greatly limits the degree of the integration of circuits.
As an attempt to overcome such a problem as mentioned above, a SWAMI (Side Wall Masked Isolation) process has been proposed. In this process, a Si.sub.3 N.sub.4 film 1 is not only provided on an active area but an Si.sub.3 N.sub.4 film 2 is provided on the side wall thereof in order to prevent the oxidation from proceeding in a transverse direction toward the active area as shown in FIG. 1a. This process is explained in an article entitled "A Bird's Beak Local Oxidation Technology Feasible for VLSI Circuits Fabrication" by K. Y. Chiu et al. in IEEE Journal of Solid-State Circuits, Vol. SC-17, No. 2 April, pp. 166, 1982.
However, as shown in FIG. 1b, such a structure entails the intrusion of the bird's beak, SiO.sub.2 region 3 beneath the silicon nitride film 2, when Si substrate 4 is oxidized to form the SiO.sub.2 region 3 at the area to be a LOCOS region by heat treatment and results in a leakage LOCOS isolation due to the generation of crystal defects at Si-SiO.sub.2 interface of LOCOS edge, and furthermore, a fragile bird's head is formed, showing concave shape around the edge of the LOCOS surface; hence, requires a part of the SiO.sub.2 region on the Si substrate surface to be eliminated and flatly ground after the LOCOS region is formed, for an optimum use of the Si.sub.3 N.sub.4 films in subsequent steps.